MIPS array indexing using a displacement in lw for a known constant index?
.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty{ height:90px;width:728px;box-sizing:border-box;
}
I am trying to translate from C this line of code of a nested array p=A[B[6]] that I've found on the book I'm studying MIPS from.
Because I am pretty sure that the solution from the book is wrong or, at least, much more complicated than what it needs to be.
The base address of the array A is stored in the register $s1, the base address of B is stored in $s2 and the value of g is stored in $s0.
My translation of p=A[B[6]] would be (please tell me if it is correct):
lw $t0, 24($s2) #load from memory B[6] in the register $t0
sll $t0, $t0, 2 #$t0=$t0*4
add $t0, $t0, $s1 #add to $t0 the address of A[B[6]]
lw $t0, 0($t0) #$t0=A[B[6]]
While instead the book (which is full of other errors) offers this solution:
addi $t0, $0, 6
sll sll $t0, $t0 2
add $t1, $s2, $t0
lw $t2, 0 ($t1)
sll $t2, $t2 2
add $t3, $s1, $t2
lw $s0, 0 ($t3)
Is my code correct or is the book right?
assembly mips micro-optimization
edited Jan 5 at 20:27
anothermh
3,36931733
asked Jan 3 at 20:30
AnnaAnna
111
add a comment |
I am trying to translate from C this line of code of a nested array p=A[B[6]] that I've found on the book I'm studying MIPS from.
Because I am pretty sure that the solution from the book is wrong or, at least, much more complicated than what it needs to be.
The base address of the array A is stored in the register $s1, the base address of B is stored in $s2 and the value of g is stored in $s0.
My translation of p=A[B[6]] would be (please tell me if it is correct):
lw $t0, 24($s2) #load from memory B[6] in the register $t0
sll $t0, $t0, 2 #$t0=$t0*4
add $t0, $t0, $s1 #add to $t0 the address of A[B[6]]
lw $t0, 0($t0) #$t0=A[B[6]]
While instead the book (which is full of other errors) offers this solution:
addi $t0, $0, 6
sll sll $t0, $t0 2
add $t1, $s2, $t0
lw $t2, 0 ($t1)
sll $t2, $t2 2
add $t3, $s1, $t2
lw $s0, 0 ($t3)
Is my code correct or is the book right?
assembly mips micro-optimization
edited Jan 5 at 20:27
anothermh
3,36931733
asked Jan 3 at 20:30
AnnaAnna
111
add a comment |
I am trying to translate from C this line of code of a nested array p=A[B[6]] that I've found on the book I'm studying MIPS from.
Because I am pretty sure that the solution from the book is wrong or, at least, much more complicated than what it needs to be.
The base address of the array A is stored in the register $s1, the base address of B is stored in $s2 and the value of g is stored in $s0.
My translation of p=A[B[6]] would be (please tell me if it is correct):
lw $t0, 24($s2) #load from memory B[6] in the register $t0
sll $t0, $t0, 2 #$t0=$t0*4
add $t0, $t0, $s1 #add to $t0 the address of A[B[6]]
lw $t0, 0($t0) #$t0=A[B[6]]
While instead the book (which is full of other errors) offers this solution:
addi $t0, $0, 6
sll sll $t0, $t0 2
add $t1, $s2, $t0
lw $t2, 0 ($t1)
sll $t2, $t2 2
add $t3, $s1, $t2
lw $s0, 0 ($t3)
Is my code correct or is the book right?
assembly mips micro-optimization
edited Jan 5 at 20:27
anothermh
3,36931733
asked Jan 3 at 20:30
AnnaAnna
111
I am trying to translate from C this line of code of a nested array p=A[B[6]] that I've found on the book I'm studying MIPS from.
Because I am pretty sure that the solution from the book is wrong or, at least, much more complicated than what it needs to be.
The base address of the array A is stored in the register $s1, the base address of B is stored in $s2 and the value of g is stored in $s0.
My translation of p=A[B[6]] would be (please tell me if it is correct):
lw $t0, 24($s2) #load from memory B[6] in the register $t0
sll $t0, $t0, 2 #$t0=$t0*4
add $t0, $t0, $s1 #add to $t0 the address of A[B[6]]
lw $t0, 0($t0) #$t0=A[B[6]]
While instead the book (which is full of other errors) offers this solution:
addi $t0, $0, 6
sll sll $t0, $t0 2
add $t1, $s2, $t0
lw $t2, 0 ($t1)
sll $t2, $t2 2
add $t3, $s1, $t2
lw $s0, 0 ($t3)
Is my code correct or is the book right?
assembly mips micro-optimization
assembly mips micro-optimization
edited Jan 5 at 20:27
anothermh
3,36931733
asked Jan 3 at 20:30
AnnaAnna
111
edited Jan 5 at 20:27
anothermh
3,36931733
asked Jan 3 at 20:30
AnnaAnna
111
edited Jan 5 at 20:27
anothermh
3,36931733
edited Jan 5 at 20:27
anothermh
3,36931733
edited Jan 5 at 20:27
anothermh
3,36931733
3,36931733
asked Jan 3 at 20:30
AnnaAnna
111
asked Jan 3 at 20:30
AnnaAnna
111
asked Jan 3 at 20:30
AnnaAnna
111
111
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
Both versions are logically correct; the only problem with the book version is that it's very inefficient.
It fails to optimize based on the fact that 6 is an assemble-time constant, so the 6*4 can be an immediate displacement in the lw, instead of calculated at runtime in a register and added to the base separately.
lw has room for 16 bits of immediate offset; it's silly not to take advantage and restrict yourself to only ever using 0. It's an I-type instruction for a reason, dedicating lots of coding space to allowing a large offset.
Other than that your versions are equivalent. The book version calculates 6<<2 in a register and adds it to the base of B ($s2). It gets the initial 6 into a register by adding to the zero-register.
Both yours and the book's use add instead of addu. Not sure why you want to trap on signed overflow, especially when doing address math. C compilers normally always use addu. (Signed overflow is undefined behaviour in C, but compiler developers know that it's usually more useful / expected for it to silently wrap than to raise an exception.)
answered Jan 3 at 20:47
Peter CordesPeter Cordes
134k18203342
add a comment |
Your Answer
StackExchange.ifUsing("editor", function () {
StackExchange.using("externalEditor", function () {
StackExchange.using("snippets", function () {
StackExchange.snippets.init();
});
});
}, "code-snippets");
StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "1"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});
function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f54029372%2fmips-array-indexing-using-a-displacement-in-lw-for-a-known-constant-index%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
Both versions are logically correct; the only problem with the book version is that it's very inefficient.
It fails to optimize based on the fact that 6 is an assemble-time constant, so the 6*4 can be an immediate displacement in the lw, instead of calculated at runtime in a register and added to the base separately.
lw has room for 16 bits of immediate offset; it's silly not to take advantage and restrict yourself to only ever using 0. It's an I-type instruction for a reason, dedicating lots of coding space to allowing a large offset.
Other than that your versions are equivalent. The book version calculates 6<<2 in a register and adds it to the base of B ($s2). It gets the initial 6 into a register by adding to the zero-register.
Both yours and the book's use add instead of addu. Not sure why you want to trap on signed overflow, especially when doing address math. C compilers normally always use addu. (Signed overflow is undefined behaviour in C, but compiler developers know that it's usually more useful / expected for it to silently wrap than to raise an exception.)
answered Jan 3 at 20:47
Peter CordesPeter Cordes
134k18203342
add a comment |
Both versions are logically correct; the only problem with the book version is that it's very inefficient.
It fails to optimize based on the fact that 6 is an assemble-time constant, so the 6*4 can be an immediate displacement in the lw, instead of calculated at runtime in a register and added to the base separately.
lw has room for 16 bits of immediate offset; it's silly not to take advantage and restrict yourself to only ever using 0. It's an I-type instruction for a reason, dedicating lots of coding space to allowing a large offset.
Other than that your versions are equivalent. The book version calculates 6<<2 in a register and adds it to the base of B ($s2). It gets the initial 6 into a register by adding to the zero-register.
Both yours and the book's use add instead of addu. Not sure why you want to trap on signed overflow, especially when doing address math. C compilers normally always use addu. (Signed overflow is undefined behaviour in C, but compiler developers know that it's usually more useful / expected for it to silently wrap than to raise an exception.)
answered Jan 3 at 20:47
Peter CordesPeter Cordes
134k18203342
add a comment |
Both versions are logically correct; the only problem with the book version is that it's very inefficient.
It fails to optimize based on the fact that 6 is an assemble-time constant, so the 6*4 can be an immediate displacement in the lw, instead of calculated at runtime in a register and added to the base separately.
lw has room for 16 bits of immediate offset; it's silly not to take advantage and restrict yourself to only ever using 0. It's an I-type instruction for a reason, dedicating lots of coding space to allowing a large offset.
Other than that your versions are equivalent. The book version calculates 6<<2 in a register and adds it to the base of B ($s2). It gets the initial 6 into a register by adding to the zero-register.
Both yours and the book's use add instead of addu. Not sure why you want to trap on signed overflow, especially when doing address math. C compilers normally always use addu. (Signed overflow is undefined behaviour in C, but compiler developers know that it's usually more useful / expected for it to silently wrap than to raise an exception.)
answered Jan 3 at 20:47
Peter CordesPeter Cordes
134k18203342
Both versions are logically correct; the only problem with the book version is that it's very inefficient.
It fails to optimize based on the fact that 6 is an assemble-time constant, so the 6*4 can be an immediate displacement in the lw, instead of calculated at runtime in a register and added to the base separately.
lw has room for 16 bits of immediate offset; it's silly not to take advantage and restrict yourself to only ever using 0. It's an I-type instruction for a reason, dedicating lots of coding space to allowing a large offset.
Other than that your versions are equivalent. The book version calculates 6<<2 in a register and adds it to the base of B ($s2). It gets the initial 6 into a register by adding to the zero-register.
Both yours and the book's use add instead of addu. Not sure why you want to trap on signed overflow, especially when doing address math. C compilers normally always use addu. (Signed overflow is undefined behaviour in C, but compiler developers know that it's usually more useful / expected for it to silently wrap than to raise an exception.)
answered Jan 3 at 20:47
Peter CordesPeter Cordes
134k18203342
answered Jan 3 at 20:47
Peter CordesPeter Cordes
134k18203342
answered Jan 3 at 20:47
Peter CordesPeter Cordes
134k18203342
answered Jan 3 at 20:47
Peter CordesPeter Cordes
134k18203342
134k18203342
add a comment |
add a comment |
Thanks for contributing an answer to Stack Overflow!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f54029372%2fmips-array-indexing-using-a-displacement-in-lw-for-a-known-constant-index%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
