Open zamazan4ik opened 6 years ago
mentioned in issue llvm/llvm-project#34959
Assuming it sticks, the next step would be to fix SimplifyCFG to propagate FMF from phi to select.
That part is at least partly done: https://reviews.llvm.org/rGebf9bf2cbc8f
But this example is harder than I imagined: we have to propagate FMF through memory ops and/or function parameters because the min/max calls take references (pointers) as arguments. That means we don't start with a phi of FP values; it's a phi of pointers to FP values.
Update - we have FMF on phi with: https://reviews.llvm.org/D67564
...but there was feedback that this may have unintended consequences, so posted for discussion on llvm-dev: http://lists.llvm.org/pipermail/llvm-dev/2019-September/135444.html
No responses so far, but as suggested, I'm waiting to build on that until people have plenty of time to see that.
Assuming it sticks, the next step would be to fix SimplifyCFG to propagate FMF from phi to select.
Current Codegen: https://godbolt.org/z/kVBRNY
All integer min/max patterns should be optimized after: https://reviews.llvm.org/rL369386
FP will have to check fast-math-flags to handle NaN and -0.0 properly. If so, we need to make sure that our FMF propagation is working as expected. In particular, we may need to extend FMF to phi nodes of FP values, so they get applied to a 'select' when we run -simplifycfg.
See this example:
#include <algorithm>
int test(float a, float b)
{
return std::max(std::min(a,b), std::max(a,b));
}
I have changed here variables types to float and optimization failed too - clang trunk with '-O3 -ffast-math'generates this:
test(float, float): # @test(float, float)
movaps xmm2, xmm1
minss xmm2, xmm0
maxss xmm1, xmm0
maxss xmm1, xmm2
cvttss2si eax, xmm1
ret
Early-cse was improved for integer min/max here: https://reviews.llvm.org/rL320640
...but we still need to match this pattern in instcombine.
define i32 @test(i32, i32) {
%3 = icmp slt i32 %1, %0
%4 = icmp slt i32 %0, %1
%5 = select i1 %3, i32 %1, i32 %0
%6 = select i1 %4, i32 %1, i32 %0
%7 = icmp slt i32 %5, %6
%8 = select i1 %7, i32 %6, i32 %5
ret i32 %8
}
Related logic added to value tracking: https://reviews.llvm.org/rL321672
We have "simplifyICmpWithMinMax" but it doesn't look for patterns like this where both operands are min and/or max.
Oops - it actually does look for these kinds of patterns after checking other cases. It's just missing some potential matches like: https://rise4fun.com/Alive/Mdm
On 3rd thought, the real problem is that early-cse doesn't know that these are equivalent: std::max(a,b) std::max(b,a) (for integers at least)
Ie, it knows that binops and compares can commute operands and be equivalent, but because min/max are not IR instructions or intrinsics, it fails to see min/max in the same way.
We have "simplifyICmpWithMinMax" but it doesn't look for patterns like this where both operands are min and/or max.
Oops - it actually does look for these kinds of patterns after checking other cases. It's just missing some potential matches like: https://rise4fun.com/Alive/Mdm
But that won't solve the 1st example. We'd have to match that as a min/max of min/max operands.
On 2nd thought, if we ignore the trailing select, this can be viewed as a failure of InstSimplify to fold the last icmp.
We have "simplifyICmpWithMinMax" but it doesn't look for patterns like this where both operands are min and/or max.
The examples here don't need -ffast-math because they're integer ops.
This might be solved by canonicalizing min/max harder in instcombine, so we 'see' the common compare instructions:
define i32 @max_of_minmax(i32 %a, i32 %b) {
%cmin = icmp slt i32 %b, %a
%cmax = icmp slt i32 %a, %b
%min = select i1 %cmin, i32 %b, i32 %a
%max = select i1 %cmax, i32 %b, i32 %a
%cmax2 = icmp slt i32 %min, %max
%max2 = select i1 %cmax2, i32 %min, i32 %max
ret i32 %max2
}
define i32 @max_of_maxmax(i32 %a, i32 %b) {
%cmax1 = icmp slt i32 %a, %b
%cmax2 = icmp slt i32 %b, %a
%max1 = select i1 %cmax1, i32 %b, i32 %a
%max2 = select i1 %cmax2, i32 %a, i32 %b
%cmax3 = icmp slt i32 %max2, %max1
%max3 = select i1 %cmax3, i32 %max2, i32 %max1
ret i32 %max3
}
Also check this case:
#include <algorithm>
int test(int a, int b)
{
return std::max(std::max(a,b), std::max(b,a));
}
Extended Description
clang(trunk) with '--std=c++17 -O3 -march=native -ffast-math' flags for this code:
generates this assembly:
gcc(trunk) with '--std=c++17 -O3 -march=native -ffast-math':
Helpful link: https://github.com/gcc-mirror/gcc/blob/07b69d3f1cd3dd8ebb0af1fbff95914daee477d2/gcc/match.pd